Coupling devices are generally known. They serve for connecting sections of medium-conveying systems to one another in a sealed fashion. To this end, a medium-conveying plug element is inserted into a support sleeve and fixed therein. The support sleeve forms part of a coupling unit and features a sliding element in its interior, as well as a sealing element that is clamped between the support sleeve and the sliding element in a release position. When the medium-conveying plug element is inserted, the sliding element is carried along by the plug element and displaced in the inserting direction such that the sealing element is released.
In a coupling device disclosed in U.S. Pat. No. 3,853,336, the sliding element adjoins and slides along the inner side of the support sleeve and is arranged between the plug element and the support sleeve in the coupled arrangement of the medium-conveying plug element in the support sleeve. In such an arrangement, a gap between the plug element and the support sleeve with a thickness corresponding to the difference between the outside diameter and the inside diameter of the sliding element can never be entirely prevented such that the tightness of this arrangement is reduced. In U.S. Pat. No. 3,853,336, two sealing elements are provided in order to ensure the tightness of the coupling unit. However, this is cumbersome and cost-intensive.